Hydraulic fracturing methods using cross-linking composition comprising delay agent

ABSTRACT

A method for hydraulically fracturing a subterranean formation comprises introducing into the formation a cross-linking composition which comprises (a) an aqueous liquid, (b) a pH buffer, (c) a cross-linkable organic polymer, (d) a cross-linking agent which comprises an organic titanate, an organic zirconate, or combinations thereof, and (e) a delay agent which is a hydroxyalkylaminocarboxylic acid. The method can be used over a wide range of pH conditions.

FIELD OF THE INVENTION

The invention relates to the field of oil well fracturing using across-linking composition. The cross-linking composition comprises across-linking agent which is a zirconium or titanium complex or mixturesthereof, a cross-linkable organic polymer and a delay agent.

BACKGROUND OF THE INVENTION

The production of oil and natural gas from an underground well(subterranean formation) can be stimulated by a technique calledhydraulic fracturing, in which a viscous fluid composition (fracturingfluid) containing a suspended proppant (e.g., sand, bauxite) isintroduced into an oil or gas well via a conduit, such as tubing orcasing, at a flow rate and a pressure which create, reopen and/or extenda fracture into the oil- or gas-containing formation. The proppant iscarried into the fracture by the fluid composition and prevents closureof the formation after pressure is released. Leak-off of the fluidcomposition into the formation is limited by the fluid viscosity of thecomposition. Fluid viscosity also permits suspension of the proppant inthe composition during the fracturing operation. Cross-linking agents,such as borates, titanates or zirconates are usually incorporated intothe composition to control viscosity.

Normally, less than one third of available oil is extracted from a wellafter it has been fractured before production rates decrease to a pointat which recovery becomes uneconomical. Enhanced recovery of oil fromsuch subterranean formations frequently involves attempting to displacethe remaining crude oil with a driving fluid, e.g., gas, water, brine,steam, polymer solution, foam, or micellar solution. Ideally, suchtechniques (commonly called flooding techniques) provide a bank of oilof substantial depth being driven into a producing well; however, inpractice this is frequently not the case. Oil-bearing strata are usuallyheterogeneous, some parts of them being more permeable than others. As aconsequence, channeling frequently occurs, so that the driving fluidflows preferentially through zones depleted of oil (so-called “thiefzones”) rather than through those parts of the strata which containsufficient oil to make oil-recovery operations profitable.

Difficulties in oil recovery due to high permeability of zones may becorrected by injecting an aqueous solution of an organic polymer and across-linking agent into certain subterranean formations underconditions where the polymer will be cross-linked to produce a gel, thusreducing the permeability of such subterranean formations to drivingfluid (gas, water, etc.). Polysaccharide- or partially hydrolyzedpolyacrylamide-based fluids cross-linked with certain aluminum,titanium, zirconium and boron-based compounds are also used in theseenhanced oil recovery applications.

Cross-linked fluids or gels, whether for fracturing a subterraneanformation or for reducing permeability of a subterranean formation, arenow being used in hotter, deeper wells under a variety of pH conditions,where rates of cross-linking with known cross-linking compositions maybe unacceptable. Rather than developing new cross-linking agents forthese new conditions, the oil well service companies may add delayagents that effectively delay the cross-linking of a particular metalcross-linking agent under these conditions.

A number of patents disclose the use of various delay agents incombination with particular cross-linking agents for which they areeffective. These patents typically specify adding one or moreingredients to a cross-linking composition or specify particularoperating conditions, such as a narrow range of pH. There are only alimited number of disclosed delay agents suitable for titanium andzirconium cross-linking agents. Thus, use of delay agents with titaniumand zirconium cross-linking agents has limited flexibility for use bythe oil well service companies to stimulate or enhance recovery of oilor gas from a well or other subterranean formation.

There is a need for a more effective method for delaying the action oftitanium and zirconium cross-linking agents in oil recoveryapplications, such as hydraulic fracturing and plugging permeable zonesand leaks. There is also a need to be able to control rate ofcross-linking in oil recovery applications so that a range ofcross-linking rates and may be achieved under a range of pH conditionswith a single cross-linking composition. The present invention meetsthese needs.

SUMMARY OF THE INVENTION

This invention provides a method for hydraulically fracturing asubterranean formation which comprises using a cross-linking compositionwhich comprises (a) an aqueous liquid, (b) a pH buffer, (c) across-linkable organic polymer, (d) a cross-linking agent whichcomprises an organic titanate, an organic zirconate, or combinationsthereof, and (e) a delay agent which is a hydroxyalkylaminocarboxylicacid. The composition can be used over a wide range of pH, especially pH3-12. Preferably the cross-linkable organic polymer is a solvatablepolysaccharide. The preferred delay agent is bishydroxyethylglycine.

This method comprises introducing the composition into a subterraneanformation at a flow rate and pressure sufficient to create, reopenand/or extend a fracture in the formation. The components of thecross-linking composition may be mixed prior to introducing them intothe formation or the components can be introduced and permitted to reactin the formation after a controllable period of time.

The present invention provides methods for effective delaying the actionof titanium and zirconium cross-linking agents in oil fieldapplications. Surprisingly, a range of temperature, pH and otherconditions can be tolerated and delay times controlled to provideflexibility by adjusting relative amounts of components, includingcross-linking agent and delay agents.

DETAILED DESCRIPTION OF THE INVENTION

This invention provides methods for use of a cross-linking composition,especially wherein the rate of cross-linking of a cross-linkable organicpolymer is delayed. These methods are useful in oil well applicationssuch as hydraulic fracturing and plugging of permeable zones.

The cross-linking composition comprises (a) an aqueous liquid; (b) a pHbuffer; (c) a cross-linkable organic polymer; (d) a cross-linking agentwhich comprises an organic titanate, an organic zirconate, orcombinations thereof; and (e) a delay agent which is ahydroxyalkylaminocarboxylic acid. The composition may further compriseproppants, stabilizers, breakers, organic solvents, and the like.

The aqueous liquid may be water, a mixture of water and an alcohol, suchas aqueous methanol or aqueous ethanol, or an aqueous solutioncomprising additional components. For example, an aqueous solution maycomprise a clay stabilizer. Clay stabilizers include, for example,hydrochloric acid and chloride salts, such as, tetramethylammoniumchloride (TMAC) or potassium chloride. Aqueous solutions comprising claystabilizers may comprise, for example, 0.05 to 0.5 weight % of thestabilizer, based on the total weight of the cross-linking composition.

The composition is useful over a wide range of pH. A pH buffer is addedto the composition to control pH. The composition may comprise a pHbuffer which is acidic, neutral or basic. The pH buffer is generallycapable of controlling the pH from about pH 3 to about pH 12. Forexample, in a composition for use at pH of about 4-5, an aceticacid-based buffer can be used. In a composition for use at a pH of 5-7,a fumaric acid-based buffer or a sodium diacetate-based buffer can beused. In a composition for use at a pH of 7-8.5, a sodiumbicarbonate-based buffer can be used. In a composition for use at a pHof 9-12, a sodium carbonate or sodium hydroxide-based buffer can beused. Other suitable pH buffers can be used, as are known to thoseskilled in the art.

Examples of suitable cross-linkable organic polymers include solvatablepolysaccharides, polyacrylamides and polymethacrylamides. Preferably theorganic polymer is a solvatable polysaccharides and is selected from thegroup consisting of gums, gum derivatives and cellulose derivatives.Gums include guar gum and locust bean gum, as well as othergalactomannan and glucomannan gums, such as those derived from sennas,Brazilwood, tera, honey locust, karaya gum and the like. Gum derivativesinclude hydroxyethylguar (HEG), hydroxypropylguar (HPG),carboxyethylhydroxyethylguar (CEHEG), carboxymethylhydroxypropylguar(CMHPG), carboxymethyl guar (CMG), and the like. Cellulose derivativesinclude those containing carboxyl groups, such as carboxymethylcellulose(CMC), carboxymethylhydroxyethylcellulose (CMHEC), and the like. Thesolvatable polysaccharides can be used individually or in combination;usually, however, a single material is used. Guar derivatives andcellulose derivatives are preferred, such as, HPG, CMC and CMHPG. HPG isgenerally more preferred based upon its commercial availability anddesirable properties. However, CMC and CMHPG may be more preferred incross-linking compositions when the pH of the composition is less than6.0 or higher than 9.0, or when the permeability of the formation issuch that one wishes to keep the residual solids at a low level toprevent damage to the formation.

The cross-linkable polymer is normally blended with a solvent such aswater or mixed water/organic solvent or with the aqueous liquid asdescribed above to form an uncross-linked gel. Organic solvents that maybe used include alcohols, glycols, polyols, and hydrocarbons such asdiesel. As an example, the polymer may be blended with water, awater/alcohol mixture (e.g., where the alcohol is methanol or ethanol),or an aqueous solution comprising a clay stabilizer).

The cross-linking agent comprises an organic titanium complex, anorganic zirconium complex or a combination thereof.

Suitable organic zirconium complexes for use in the composition of thisinvention include but are not limited to zirconium α-hydroxycarboxylicacid salt, zirconium polyol complexes, zirconium alkanol aminecomplexes, zirconium hydroxyalkylated alkylenediamine complexes, andcombinations thereof. Examples of useful zirconium complexes includezirconium diethanolamine complex, zirconium triethanolamine complex,zirconium lactate, zirconium ethylene glycolate, zirconiumacetylacetonate, zirconium ammonium lactate, zirconium diethanolaminelactate, zirconium triethanolamine lactate, zirconium diisopropylaminelactate, zirconium sodium lactate salts, zirconium glycerol complex,zirconium sorbitol complex, zirconium hydroxyalkylated ethylenediaminecomplexes, or combinations thereof.

The preferred zirconium complexes are zirconium polyol complexes andzirconium alkanol amine complexes. Polyols include glycerol, erythritol,threitol, ribitol, arabinitol, xylitol, allitol, altritol, sorbitol,mannitol, dulcitol, iditol, perseitol, and the like. Alkanol aminesinclude those corresponding to the formula R′—N—CH₂—CH(OH)R″)₂ whereinR′ is hydrogen or —CH₂—CH(OH)R″ and R″ is hydrogen, methyl or ethyl. Amore preferred zirconium complex is zirconium tetra-triethanolaminecomplex, which is available commercially from E. I. du Pont de Nemoursand Company, Wilmington, Del., as Tyzor® TEAZ organic zirconate.

Suitable organic titanium complexes for use in the composition of thisinvention include but are not limited to titanium α-hydroxycarboxylicacid salt, titanium polyol complexes, titanium alkanol amine complexes,and combinations thereof. Examples of useful titanium complexes includetitanium diethanolamine complex, titanium triethanolamine complex,titanium lactate, titanium ethylene glycolate, titanium acetylacetonate,titanium ammonium lactate, titanium diethanolamine lactate, titaniumtriethanolamine lactate, titanium diisopropylamine lactate, titaniumsodium lactate salts, titanium sorbitol complexes, and combinationsthereof.

The preferred titanium complexes are titanium alkanol amine complexes.Suitable alkanol amines are those described hereinabove. A morepreferred titanium complex is titanium tetra-triethanolamine complex,which is available commercially from E. I. du Pont de Nemours andCompany, Wilmington, Del. as Tyzor® TE organic titanate.

The cross-linking agent is generally used as a solution or suspension inan organic, aqueous or mixed aqueous/organic solvent. Organic solventsare typically alcohols, such as ethanol, n-propanol, i-propanol, and thelike. For example, the cross-linking agent can be used as a solution inthe aqueous liquid. The concentration of the cross-linking agent canvary and is typically from 0.01 to 1.0 weight %, based on the totalweight of the cross-linking composition. The preferred concentration is0.1 to 0.5 weight %, based on the total weight of the composition.

The delay agent is a hydroxyalkylaminocarboxylic acid. Preferably, thedelay agent is selected from the group consisting ofbishydroxyethylglycine, bishydroxymethylglycine,bishydroxypropylglycine, bishydroxyisopropylglycine,bishydroxybutylglycine, monohydroxyethylglycine,monohydroxymethylglycine and their alkali metal salts. More preferablythe hydroxyalkylaminocarboxylic acid is bishydroxyethylglycine.

The delay agents are commercially available and/or may be prepared byprocesses described in the literature. For example,bishydroxyethylglycine suitable for this invention may be made by anumber of processes described in the literature (see, Kromov-Borisov andRemizov, in Zhur. Obshchei Khim., 1953, 23, 598; Gump, et al., in J.Org. Chem., 1959, 24, 712-14). Bishydroxyethylglycine is also availablecommercially and sold under the generic name of “bicine”.

The delay agent is typically used as an aqueous solution. Theconcentration of delay agent in the solution may vary and is typicallyfrom 0.1-75% by weight. The preferred concentration is 10-30 weight %,based on the total weight of the solution.

The composition may comprise optional components, including those whichare common additives for oil field applications. Thus, the compositionmay further comprise one or more of proppants, friction reducers,bactericides, hydrocarbons, chemical breakers, stabilizers, surfactants,formation control agents, and the like. Proppants include sand, bauxite,glass beads, nylon pellets, aluminum pellets and similar materials.Friction reducers include polyacrylamides. Hydrocarbons include dieseloil. Chemical breakers break the cross-linked polymer (gel) in acontrolled manner and include enzymes, alkali metal persulfate, ammoniumpersulfate. Stabilizers include methanol, alkali metal thiosulfate,ammonium thiosulfate. Stabilizers may also include clay stabilizers suchas hydrochloric acid and chloride salts, for example,tetramethylammonium chloride (TMAC) or potassium chloride.

The composition may also further comprise as optional components, acomplexing agent or a polyfunctional organic compound, such as one ormore of hydroxycarboxylic acid, aminocarboxylic acid, alkanolamine(hydroxyalkylamines, hydroxyalkyl alkylenediamines, polyhydroxylcompounds, sodium carbonate, and sodium bicarbonate. Hydroxycarboxylicacid includes polyhydroxylcarboxylic acid, hydroxy monocarboxylic acid,α-hydroxycarboxylic acid. Polyhydroxyl compounds include polyols andpolyhydroxylcarboxylic acids.

Each component is present in the composition in an amount sufficient toachieve the desired cross-linking performance based on the individualcomponents, desired delay in cross-linking time, temperature and otherconditions present in the formation being fractured or permeable zonebeing plugged. Aqueous liquid is added in an amount sufficient to renderthe composition active for cross-linking the cross-linkable polymer bythe cross-linking agent in the presence of the delay agent. The pHbuffer is added in an amount sufficient to maintain pH of thecomposition in the desired pH range.

The amounts of cross-linkable polymer and the cross-linking agent mayvary. One uses small but effective amounts, each of which will vary withthe circumstances, e.g., the type of subterranean formation, the depthat which the method (e.g., fluid fracturing, permeable zone plugging orleak plugging) is to be performed, as well as temperature and pH, amongother conditions. Generally one uses as small an amount of each as willprovide the viscosity level necessary to effect the desired result,i.e., fracturing of the subterranean formation, or plugging of permeablezones or leaks in order to promote adequate recovery of oil or gas froma subterranean formation.

The amount of delay agent is dependent on the extent to which the rateof cross-linking is desired to be delayed. Typically the ratio of thedelay agent to cross-linking agent, on a weight basis, is 0.001:1 to100:1 of delay agent to cross-linking agent. Preferably when the delayagent is bishydroxyethylglycine, this ratio is 0.1-10:1 of delay agentto cross-linking agent. Within these broad ranges, the amount of delayagent selected for use is dependent on the type and amount ofcross-linking agent being used, the temperature of the formation beingfractured or permeable zone being plugged and the delay in cross-linktime desired. As the weight ratio of delay agent to cross-linking agentis increased, the rate of cross-linking, i.e., gel formation is reducedor cross-link time is increased. At higher ratios of delay agent tocross-linking agent, higher temperature may be needed to initiatecross-linking. The maximum viscosity of the final gel decreases ascross-link times are increased. By controlling the rate of cross-linkingof the polymer by the use of a delay agent in combination with a singlecross-linking agent over the variety of pH and temperature conditionsexperienced in the field, one can minimize premature cross-linking onthe surface and subsequent viscosity loss due to shear degradation.

The composition of this invention may be produced by mixing the aqueousliquid, pH buffer, organic polymer, cross-linking agent and delay agent,along with any optional components in any order. For example, in aparticular application in the oil field, the components may beintroduced into a subterranean formation as separate streams, or two ormore of the components may be premixed and introduced into the formationas a combined stream, or all components may be premixed and introducedas a single stream. Preferably, the cross-linkable polymer is notpremixed with the cross-linking agent. When these two components arepremixed, they are premixed just prior to the use of the composition,that is, introducing the mixture into a subterranean formation, forexample, for hydraulic fracturing or plugging of subterranean permeablezones or leaks. Advantageously, the components may be mixed in differentcombinations, and more advantageously, the components may be mixed justprior to use to enable easy variation and adjustment of thecross-linking rate.

The compositions of this invention provide advantages over those of theprior art when used in methods for hydraulic fracturing or plugging ofsubterranean zones or leaks. The compositions can be modified to providea range of cross-linking rates with a single cross-linking agent. Thecompositions can be used at both high and low pH. The compositions canbe used at high temperatures at acceptable rates. The compositions canbe used with fluids containing a high level of brine. Thus, thecompositions can be used in hot subterranean formations, including thoseat greater depths in oil and gas wells. The compositions provideexcellent performance in hydraulic fracturing and for selectivelyplugging permeable zones and leaks in subterranean formations.

The present invention further provides methods of using thecross-linking composition of this invention. In a hydraulic fracturingmethod of this invention, one or more fractures is created, reopened,and/or extended in an oil- or gas-containing subterranean. Thus, thisinvention provides a method for fracturing a subterranean formationwhich comprises introducing into said formation a cross-linkingcomposition at a flow rate and pressure sufficient to create, reopenand/or extend a fracture in said formation, wherein said compositioncomprises (a) an aqueous liquid, (b) a pH buffer, (c) a cross-linkableorganic polymer, (d) a cross-linking agent which comprises an organictitanate, an organic zirconate, or combinations thereof, and (e) a delayagent which is a hydroxyalkylaminocarboxylic acid.

In a first embodiment of the method for hydraulically fracturing asubterranean formation, the cross-linkable organic polymer and thecross-linking agent are contacted prior to their introduction into thesubterranean formation, such that the polymer and cross-linking agentreact to form a cross-linked aqueous gel, which gel is then introducedinto the formation.

In one example of the first embodiment of the hydraulic fracturingmethod, a base gel is prepared by mixing an aqueous liquid with across-linkable organic polymer and a delayed cross-linking compositionis prepared by mixing a cross-linking agent which comprises an organictitanate, an organic zirconate, or combinations thereof, with a delayagent which is a hydroxyalkylaminocarboxylic acid. A pH buffer is addedto the base gel, the delayed cross-linking composition, or both. In thisembodiment, more specifically, the method for hydraulically fracturing asubterranean formation comprises (a) preparing a base gel; (b) preparinga delayed cross-linking composition; (c) contacting the base gel withthe delayed cross-linking composition; (d) permitting the base gel andthe cross-linking agent to react after a controllable amount of time toform a cross-linked aqueous gel; and (e) introducing the cross-linkedgel into the formation at a flow rate and pressure sufficient to create,reopen, and/or extend a fracture in the formation.

In a second example of the first embodiment, a base gel is prepared bymixing an aqueous liquid with a cross-linkable polymer and a delay agentwhich is a hydroxyalkylaminocarboxylic acid. In this embodiment, themethod for hydraulically fracturing a subterranean formation comprises(a) preparing a base gel; (b) contacting the base gel with across-linking agent which comprises an organic titanate, an organiczirconate, or combinations thereof; (c) permitting the base gel and thecross-linking agent to react after a controllable amount of time to forma cross-linked aqueous gel; and (d) introducing the cross-linked gelinto the formation at a flow rate and pressure sufficient to create,reopen, and/or extend a fracture in the formation. In this secondembodiment, a pH buffer is admixed with the base gel, the cross-linkingagent, or both, prior to contacting the base gel with the cross-linkingagent.

In a modification of this first embodiment, the subterranean formationmay be penetrated by a wellbore, such that contacting the base gel withthe cross-linking agent occurs in the wellbore and the cross-linkedaqueous gel is introduced into the formation from the wellbore at a flowrate and pressure sufficient to create, reopen and/or extend a fracturein the formation.

In a second embodiment, components of a cross-linking composition areintroduced separately, either sequentially or simultaneously, into asubterranean formation such that cross-linking occurs within thesubterranean formation. The method of this embodiment for hydraulicallyfracturing a subterranean formation penetrated by a wellbore comprises(a) preparing a base gel by mixing an aqueous liquid with across-linkable polymer; (b) introducing the base gel into the wellbore;(c) simultaneously with or sequentially after, introducing the base gelinto the wellbore, introducing a cross-linking agent which comprises anorganic titanate, an organic zirconate, or combinations thereof into thewellbore; wherein a pH buffer and a delay agent which is ahydroxyalkylaminocarboxylic acid are independently admixed with the basegel, the cross-linking agent or both prior to introducing the base geland the cross-linking agent into the wellbore; (d) permitting the basegel and the cross-linking agent to react after a controllable period oftime to form a cross-linked aqueous gel; and (e) introducing thecross-linked gel into the formation from the wellbore at a flow rate andpressure sufficient to create, reopen, and/or extend a fracture in theformation.

Upon creation of a fracture or fractures, the method may furthercomprise introducing a cross-linking composition comprising (a) anaqueous liquid, (b) a pH buffer, (c) a cross-linkable organic polymer,(d) a cross-linking agent which comprises an organic titanate, anorganic zirconate, or combinations thereof, (e) a delay agent which is ahydroxyalkylaminocarboxylic acid and (f) proppant, into the fracture orfractures. This second introduction of a cross-linking composition ispreferably performed in the event the cross-linking composition used tocreate the fracture or fractures did not comprise proppant. Thecross-linking composition may subsequently be recovered from theformation.

In the method for fracturing a subterranean formation, satisfactory gelscan generally be made by using the cross-linkable organic polymer inamounts up to about 1.2 weight % and the cross-linking agent in amountsup to about 1.0 weight %, both percentages being based on the weight ofthe aqueous liquid. Preferably, from about 0.25 to about 0.75 weight %of the cross-linkable organic polymer is used and from about 0.05 toabout 0.50 weight % of the cross-linking agent is used, both percentagesbeing based on the weight of the aqueous liquid.

In another method of this invention, the composition of this inventionis used to plug a permeable zone or leak in a subterranean formation.This method comprises introducing a cross-linking composition (orcross-linked gel) into the permeable zone or leak.

More specifically, the method of plugging a permeable zone or a leak ina subterranean formation comprises introducing into said zone or saidleak, a cross-linking composition comprising (a) an aqueous liquid, (b)a pH buffer, (c) a cross-linkable organic polymer, (d) a cross-linkingagent which comprises an organic titanate, an organic zirconate, orcombinations thereof, and (e) a delay agent which is ahydroxyalkylaminocarboxylic acid.

In a first embodiment of the method for plugging a permeable zone or aleak in a subterranean formation, the cross-linkable organic polymer andthe cross-linking agent are contacted prior to their introduction intothe subterranean formation, such that the polymer and cross-linkingagent react to form a cross-linked aqueous gel, which gel is thenintroduced into the formation.

In one example of the first embodiment of the plugging a permeable zoneor a leak in a subterranean formation method, a base gel is prepared bymixing an aqueous liquid with a cross-linkable organic polymer and adelayed cross-linking composition is prepared by mixing a cross-linkingagent which comprises an organic titanate, an organic zirconate, orcombinations thereof, with a delay agent which is ahydroxyalkylaminocarboxylic acid. A pH buffer is added to the base gel,the delayed cross-linking composition, or both. In this embodiment, morespecifically, the method comprises (a) preparing the base gel; (b)preparing a delayed cross-linking composition; (c) contacting the basegel with the delayed cross-linking composition; (d) permitting the basegel and the cross-linking agent to react after a controllable amount oftime to form a cross-linked aqueous gel; and (e) introducing thecross-linked gel into the permeable zone or leak.

In a second example of the first embodiment, a base gel is prepared bymixing an aqueous liquid with a cross-linkable polymer and a delay agentwhich is a hydroxyalkylaminocarboxylic acid. In this embodiment, themethod for plugging a permeable zone or leak comprises (a) preparing thebase gel; (b) contacting the base gel with a cross-linking agent whichcomprises an organic titanate, an organic zirconate, or combinationsthereof; (d) permitting the base gel and the cross-linking agent toreact after a controllable amount of time to form a cross-linked aqueousgel; and (e) introducing the cross-linked gel into the permeable zone orleak. In this second embodiment, a pH buffer is added to the base gel oradmixed with cross-linking agent.

In a second embodiment, components of a cross-linking composition areintroduced separately into a permeable zone or leak in a subterraneanformation such that cross-linking occurs within the subterraneanformation. The method of this embodiment comprises (a) preparing a basegel by mixing an aqueous liquid with a cross-linkable polymer; (b)introducing the base gel into the permeable zone or leak; (c)simultaneously with or sequentially after, introducing the base gel intothe permeable zone or leak, introducing a cross-linking agent whichcomprises an organic titanate, an organic zirconate, or combinationsthereof into permeable zone or leak; wherein a pH buffer and a delayagent which is a hydroxyalkylaminocarboxylic acid are independentlyadmixed with the base gel, the cross-linking agent or both prior tointroduction of the base gel and the cross-linking agent into thepermeable zone or leak; and (d) permitting the base gel and thecross-linking agent to react after a controllable period of time to forma cross-linked aqueous gel to plug the permeable zone or leak.

In a method for plugging permeable zones or leaks in subterraneanformations, one generally uses about 0.25 to 1.2 weight % of across-linkable organic polymer, preferably 0.40 to 0.75 weight %, and0.01 to 1.0 weight % of a cross-linking agent, preferably 0.05 to 0.50weight %, all percentages being based on the weight of the aqueousliquid.

EXAMPLES Methods

Preparation of a Base Gel

One liter of tap water was added to a Waring blender jar equipped with athree bladed paddle stirrer. Agitation was started and 3.6 g of asolvatable polysaccharide polymer was added, followed by a claystabilizer (tetramethylammonium chloride) and a buffer selected toadjust the pH to 4.0-7.0 to provide a solution. The rate of agitationwas adjusted to maintain a slight vortex at the top of the solution andagitation continued for 30 minutes, which produced a “30 lb/1000 gallon”base gel. After 30 minutes, the pH of the base gel was adjusted to thedesired final pH with (1) an acetic acid-based buffer for pH 4-5; (2) afumaric acid or sodium diacetate-based buffer for pH 5-7; (3) a sodiumbicarbonate-based buffer for pH 7-8.5; or (4) a sodium carbonate orsodium hydroxide-based buffer for pH 9-11. Agitation was stopped and thebase gel allowed to sit for 30 minutes.

Alternatively, for a “20 lb/1000” gallon base gel, 2.4 g of polymer wasadded to one liter of tap water. For a “60 lb/1000” gallon base gel, 7.2g of polymer was added to one liter of tap water.

Vortex Closure Test:

A 250 ml portion of base gel was measured into a clean Waring blenderjar. Agitation was started and the rate adjusted to create a vortexexposing the blade nut. The setting on the Variac controlling theblender speed was recorded and kept constant for all tests forreproducibility. An amount of cross-linking agent was injected into theedge of the vortex of the agitated base gel and a stopwatch immediatelystarted, which set time=0. When the viscosity of the gel increasedsufficiently to allow the fluid to cover the nut on the blade of theblender jar and the vortex remained closed, the time was recorded. Thistime, that is the difference between the time the stopwatch started andthe time the vortex remained closed, is the vortex closure time. If thevortex had not closed within 10 minutes, the test was discontinued and avortex closure time of greater than 10 minutes was recorded. Thebeginning and final pH of the cross-linked gel were also recorded as pHband pHf, respectively. Such vortex closure tests provide a means forobtaining a reasonably good estimate of the time required to completecross-linking of the polymer by the cross-linking agent. Completeclosure of the vortex indicates a substantial degree of cross-linking.

The test was repeated using the same base gel and cross-linking agent.However, a specified amount of bishydroxyethylglycine delay agent wasinjected immediately following the injection of the cross-linking agent.The vortex closure time was recorded in a similar fashion. Results forthe cross-linking compositions are provided below.

Note 1: 0.2% by weight of the total composition of tetramethyl ammoniumchloride was used as clay stabilizer.

Note 2: A 30 lb/1000 gallon carboxymethylcellulose (CMC) base gel,prepared in 1 gal/1000 gal of 50% TMAC solution in water was used tomeasure the vortex closure times at pH 4.

Note 3: A 20 lb/1000 gal carboxymethylcellulose (CMC) base gel, preparedin 1 gal/1000 gal of 50% TMAC solution in water was used to measure thevortex closure times at pH 5.

Note 4: A 60 lb/1000 gal carboxymethylhydroxypropylguar (CMHPG) base gelwas used to measure the vortex closure times at pH 10.

Example 1

Sodium chloroacetate (237 g) was added to 422 g of tap water in a2-liter flask equipped with a dropping funnel, thermocouple, condenserand nitrogen bubbler. Agitation was started and heat applied to dissolvethe sodium chloroacetate. After the sodium chloroacetate dissolved, 218g of diethanolamine (99%) were added, and the reaction mass heated toreflux and held there for 10 hours. On cooling the solution was dilutedwith 510 g of water to give a clear, water white solution containing 24%bishydroxyethylglycine. The product of Example 1 was evaluated as adelay agent with each of the products of Examples 2-5 and ComparativeExample D.

Example 2

A 500-ml flask, equipped with a thermocouple, dropping funnel, nitrogenbleed and condenser, was charged with 313.7 g of zirconiumtetra-triethanolamine complex, available from E. I. du Pont de Nemoursand Company, Wilmington, Del. Agitation was started and a mixture of20.9 g of glycerol and 20.9 g of water were added. The solution wasagitated for 2 hours at 60° C. to give 355 g of an orange solutioncontaining 11.6% Zr. Table 1A provides results using the product ofExample 2 in the Vortex Closure Test.

Example 3

A 500-ml flask, equipped with a thermocouple, dropping funnel, nitrogenbleed and condenser, was charged with 313.7 g of zirconiumtetra-triethanolamine complex. Agitation was started and the followingwere added: 132.6 g of Quadrol®tetrakis(2-hydroxypropyl)ethylenediamine, available from BASF Corp., anda mixture of 42 g of glycerol and 42 g of water. The solution wasagitated for 2 hours at 60° C. to give 530 g of an orange solutioncontaining 7.8% Zr. Table 1B provides results using the product ofExample 3 in the Vortex Closure Test.

Comparative Example A

A 1000-ml flask equipped with agitator, a condenser, a dropping funnel,a thermocouple and a nitrogen bleed was charged with 352 g (0.799 mol)of tetra-n-propylzirconate. Agitation was started and 230.8 g (0.83 mol)of hydroxyethyl tris-(2-hydroxypropyl)ethylenediamine were added. Thereaction mass was heated to 60° C. and held there for 2 hours. After thehold period the reaction mass was cooled to room temperature to yield aviscous, clear yellow liquid containing 12.3% Zr. Table 1C providesresults using the product of Comparative Example A in the Vortex ClosureTest.

Comparative Example B

A 1000-ml flask equipped with agitator, a condenser, a dropping funnel,a thermocouple and a nitrogen bleed, was charged with 364 g (0.826 mol)of tetra-n-propylzirconate. Agitation was started and 493.4 g (3.3 mol)of triethanolamine were added. The reaction mass was heated to 60° C.and held there for 2 hours. After the hold period, a 20 mm Hg vacuum wasapplied to remove the n-propanol liberated in the reaction. The reactionmass was then cooled to room temperature to yield a viscous, clearyellow liquid containing 13.2% Zr. Table 1C provides results using theproduct of Comparative Example B in the Vortex Closure Test.

Comparative Example C

A 1000-ml flask equipped with agitator, a condenser, a dropping funnel,a thermocouple and a nitrogen bleed, was charged with 368.6 g (0.609mol) of zirconium oxychloride, dissolved as 30% aqueous solution.Agitation was started and 40 g (0.83 mol) of water were added. Next,181.3 g (1.77 mol) of 85% lactic acid were rapidly added under highspeed agitation, while temperature was maintained at 20-30° C. Thereaction mass was stirred an additional hour at 20-30° C. and thenneutralized to pH 6.7-7.3 with 25% aqueous sodium hydroxide solution.The reaction mass was then heated to 80° C. and held there for 4 hours.After the hold period the reaction mass was cooled to room temperatureto yield a clear, pale yellow liquid containing 5.4% Zr. Table 1Cprovides results using the product of Comparative Example C in theVortex Closure Test. TABLE 1A Vortex Vortex Conc Closure Closure Cross-(ml/ Time Time linking 1000 Conc (ml/ (min:sec) (min:sec) Agent ml)Delay Agent 1000 ml) pH 4 pH 5 Example 2 0.35 none 0 1:10 Example 2 0.35Example 1 1 2:05 Example 2 0.35 glycerol (70%) 1 1:22 Example 2 0.35sorbitol (70%) 1 1:04 Example 2 0.70 Example 1 1 2:39 Example 2 0.70none 0 0:59 Example 2 0.70 glycerol (70%) 1 2:01 Example 2 0.70 sorbitol(70%) 1 1:05

TABLE 1B Vortex Vortex Conc Closure Closure Cross- (ml/ Time Timelinking 1000 Conc (ml/ (min:sec) (min:sec) Agent ml) Delay Agent 1000ml) pH 4 pH 5 Example 3 0.50 None 0 1:14 Example 3 0.50 Example 1 1 3:03Example 3 0.50 glycerol (70%) 1 1:42 Example 3 0.50 sorbitol (70%) 11:18 Example 3 1.0 None 1:23 Example 3 1.0 Example 1 1 4:49 Example 31.0 glycerol (70%) 1 2:39 Example 3 1.0 sorbitol (70%) 1 1:33 Example 30.50 Example 1 0 3:50 Example 3 0.50 Example 1 0.5 7:45 Example 3 0.50Example 1 1 >10 Example 3 0.75 Example 1 0 1:44 Example 3 0.75 Example 10.5 5:29 Example 3 0.75 Example 1 1 >10

TABLE 1C Vortex Vortex Closure Closure Time Time Cross-linking Conc (ml/Delay Conc (ml/ (min:sec) (min:sec) Agent 1000 ml) Agent 1000 ml) pH 4pH 5 Comp. Ex. A. 0.4 None — 0:02 0:23 Comp. Ex. B 0.08 None — >10 Comp.Ex. B 0.12 none — 0:32 Comp. Ex. C 0.20 None — 6:17 Comp. Ex. C 0.28None — >10

Tables 1A-1C provide the evaluation results for the vortex closure timeswhen using cross-linking compositions comprising different delay agents,including compositions comprising the products of Examples 1 and 2 andof Comparative Examples A, B and C, at pH 4 and at pH 5. From Tables1A-1C, it can be seen that bishydroxyethylglycine is a much moreeffective delay agent at pH 4 and pH 5 than glycerol and sorbitol, whichare delay agents disclosed in the prior art.

Table 1B illustrates the effect of increasing the delay agentconcentration on rate of cross-linking. That is, higher amounts of delayagent increase rate of cross-linking.

In order to meet the performance requirements for use in a low pHfracturing fluid cross-link times at either pH 4 or pH 5 shouldtypically be within a period of time of 2 seconds to 5 minutes. Thevortex closure times of Comparative Examples, which lack thebishydroxyethylglycine delay agent are outside of this time period.

Comparative Example D

The effect of various delay agents in combination with a boron compound,boric acid, as a cross-linking agent were determined using the VortexClosure Test as described above. Equimolar amounts of delay agent andboric acid (0.15 g) were added to a 30 lb/100 gallon CMHPG base gel inwhich pH was adjusted to about pH 12 using sodium hydroxide. The vortexclosure times in minutes are provided in Table 2. TABLE 2 Rate ofCross-linking of Boric Acid with Bishydroxyethylglycine Vortex ClosureDelay Agent (amount added) Time (min.) pHb pHf No Delay Agent 0:48 12.9012.59 Sodium Glutamate (0.75 g) 6:13 12.90 12.57 Sorbitol (0.85 g) >1012.95 12.58 Example 1 (2.67 g) 0:53 12.95 12.60 Example 1 (5.37 g) 0:3712.97 12.58

As can be seen from Table 2, use of the composition of Example 1,bishydroxyethylglycine, is a poor delay agent for the boroncross-linking agent. The vortex closure time when using boric acid as across-linking agent is substantially the same with or without additionof bishydroxyethylglycine. In contrast, other known delay agents (sodiumglutamate and sorbitol) are effective at increasing the vortex closuretime when used with the boron cross-linking agent.

Example 4

A 500-ml flask, equipped with a thermocouple, dropping funnel, nitrogenbleed and condenser, was charged with 313.7 g of zirconiumtetra-triethanolamine complex. Agitation was started and the followingwere added to the flask: 132.6 g of Quadrol®tetrakis(2-hydroxypropyl)ethylenediamine and a mixture of 42 g ofglycerol and 21 g of water. The solution was agitated for 2 hours at 60°C. to give 509 g of an orange solution containing 8.1% Zr.

The product was evaluated in the Vortex Closure Time test along with acommercially available zirconate cross-linking agent, zirconiumtetra-triethanolamine complex, available from E. I. du Pont de Nemoursand Company, Wilmington, Del. Each cross-linking agent was used in anequimolar amount. A 60 lb/1000 gal CMHPG base gel prepared as describedabove under Preparation of a Base Gel was used. Sodium hydroxide wasused to provide a pH of 10. A test was performed in the absence and thenin the presence of bishydroxyethylglycine, the product of Example 1.TABLE 3 Conc. Cross- Vortex linking Conc. Closure Agent Example 1 TimeCross-linking Agent ml/1000 ml ml/1000 ml (min.) pHb pHf Example 4 1.080 7:24 10.00 10.04 Example 4 1.08 0.25 >10 10.00 9.80 Zirconium tetra-0.68 0 1:52 10.00 10.00 triethanolamine complex Zirconium tetra- 0.680.25 8:47 10.00 9.84 triethanolamine complex

Table 3 shows that bishydroxyethylglycine is very effective at pH 10 indelaying the rate by cross-linking of zirconate complexes such as thezirconium complex prepared in Example 4 or a commercial zirconiumcross-linking agent, zirconium tetra-triethanolamine complex.

Example 5

Two commercially available titanium cross-linking agents, titaniumtriethanolamine complex (available as Tyzor® TE organic titanate) andtitanium ammonium lactate (available as Tyzor® LA organic titanate),both from E. I. du Pont de Nemours and Company, Wilmington, Del., wereevaluated in the Vortex Closure Time test. Each cross-linking agent wasused in an amount of 0.52 ml per 1000 ml of solution of the 60 lb/1000gal CMHPG prepared as described above under Preparation of a Base Gel.Sodium hydroxide was used to provide a pH of 10. A test was performed inthe absence and then in the presence of bishydroxyethylglycine, theproduct of Example 1. TABLE 4 Conc. Example 1 Vortex ClosureCross-linking Agent (ml/1000 ml) Time (min.) Titanium triethanolaminecomplex 0 1:06 Titanium triethanolamine complex 0.25 4:12 Titaniumammonium lactate 0 4:01 Titanium ammonium lactate 0.25 >10

Table 4 shows that bishydroxyethylglycine is very effective at pH 10 indelaying the rate of cross-linking by titanate complexes.

1. A method for fracturing a subterranean formation which comprisesintroducing into said formation a cross-linking composition at a flowrate and pressure sufficient to create, reopen and/or extend a fracturein said formation, wherein said composition comprises (a) an aqueousliquid, (b) a pH buffer, (c) a cross-linkable organic polymer, (d) across-linking agent which comprises an organic titanate, an organiczirconate, or combinations thereof, and (e) a delay agent which is ahydroxyalkylaminocarboxylic acid.
 2. The method of claim 1 wherein thecross-linkable organic polymer is a solvatable polysaccharide selectedfrom gums, gum derivatives, and cellulose derivatives.
 3. The method ofclaim 2 wherein the delay agent is selected from the group consisting ofbishydroxyethylglycine, bishydroxymethylglycine,bishydroxypropylglycine, bishydroxyisopropylglycine,bishydroxybutylglycine, monohydroxyethylglycine,monohydroxymethylglycine and their alkali metal salts.
 4. The method ofclaim 3 wherein the delay agent is bishydroxyethylglycine.
 5. The methodof claim 3 wherein the cross-linking agent is an organic zirconiumcomplex selected from the group consisting of zirconiumα-hydroxycarboxylic acid salt, zirconium polyol complexes, zirconiumalkanol amine complexes, zirconium hydroxyalkylated alkylenediaminecomplexes, and combinations thereof.
 6. The method of claim 3 whereinthe cross-linking agent is an organic titanium complex selected from thegroup consisting of titanium α-hydroxycarboxylic acid salt, titaniumpolyol complexes, titanium alkanol amine complexes, and combinationsthereof.
 7. A method for fracturing a subterranean formation whichcomprises (a) preparing a base gel by mixing an aqueous liquid with across-linkable organic polymer; (b) preparing a delayed cross-linkingcomposition by mixing a cross-linking agent which comprises an organictitanate, an organic zirconate, or combinations thereof with a delayagent which is a hydroxyalkylaminocarboxylic acid; wherein a pH bufferis added to the base gel, the delayed cross-linking composition or both;(c) contacting the base gel with the delayed cross-linking composition;(d) permitting the base gel and the cross-linking agent to react after acontrollable amount of time to form a cross-linked aqueous gel; and (e)introducing the cross-linked gel into the formation at a flow rate andpressure sufficient to create, reopen, and/or extend a fracture in theformation.
 8. The method of claim 7 wherein the subterranean formationis penetrated by a wellbore and wherein said contacting step (c) occursin the wellbore.
 9. The method of claim 8 wherein the delay agent isbishydroxyethylglycine.
 10. The method of claim 9 wherein thecross-linking agent is an organic zirconium complex selected from thegroup consisting of zirconium α-hydroxycarboxylic acid salt, zirconiumpolyol complexes, zirconium alkanol amine complexes, zirconiumhydroxyalkylated alkylenediamine complexes, and combinations thereof.11. The method of claim 9 wherein the cross-linking agent is an organictitanium complex selected from the group consisting of titaniumα-hydroxycarboxylic acid salt, titanium polyol complexes, titaniumalkanol amine complexes, and combinations thereof.
 12. A method forfracturing a subterranean formation which comprises: (a) preparing abase gel by mixing an aqueous liquid with a cross-linkable organicpolymer and a delay agent which is a hydroxyalkylaminocarboxylic acid;(b) contacting the base gel with a cross-linking agent which comprisesan organic titanate, an organic zirconate, or combinations thereof;wherein a pH buffer is admixed with the base gel, the cross-linkingagent or both, prior to contacting; (c) permitting the base gel and thecross-linking agent to react after a controllable amount of time to forma cross-linked aqueous gel; and (d) introducing the cross-linked gelinto the formation at a flow rate and pressure sufficient to create,reopen, and/or extend a fracture in the formation.
 13. The method ofclaim 12 wherein the subterranean formation is penetrated by a wellboreand wherein said contacting step (b) occurs in the wellbore.
 14. Themethod of claim 13 wherein the delay agent is bishydroxyethylglycine.15. The method of claim 14 wherein the cross-linking agent is an organiczirconium complex selected from the group consisting of zirconiumα-hydroxycarboxylic acid salt, zirconium polyol complexes, zirconiumalkanol amine complexes, zirconium hydroxyalkylated alkylenediaminecomplexes, and combinations thereof.
 16. The method of claim 14 whereinthe cross-linking agent is an organic titanium complex selected from thegroup consisting of titanium α-hydroxycarboxylic acid salt, titaniumpolyol complexes, titanium alkanol amine complexes, and combinationsthereof.
 17. A method for hydraulically fracturing a subterraneanformation penetrated by a wellbore which comprises: (a) preparing a basegel by mixing an aqueous liquid with a cross-linkable polymer; (b)introducing the base gel into the wellbore; (c) simultaneously with orsequentially after, introducing the base gel into the wellbore,introducing a cross-linking agent which comprises an organic titanate,an organic zirconate, or combinations thereof into the wellbore; whereina pH buffer and a delay agent which is a hydroxyalkylaminocarboxylicacid are independently admixed with the base gel, the cross-linkingagent or both prior to introducing the base gel and the cross-linkingagent into the wellbore; (d) permitting the base gel and thecross-linking agent to react after a controllable period of time to forma cross-linked aqueous gel; and (e) introducing the cross-linked gelinto the formation from the wellbore at a flow rate and pressuresufficient to create, reopen, and/or extend a fracture in the formation.18. The method of claim 17 wherein the delay agent isbishydroxyethylglycine.
 19. The method of claim 18 wherein thecross-linking agent is an organic zirconium complex selected from thegroup consisting of zirconium α-hydroxycarboxylic acid salt, zirconiumpolyol complexes, zirconium alkanol amine complexes, zirconiumhydroxyalkylated alkylenediamine complexes, and combinations thereof.20. The method of claim 18 wherein the cross-linking agent is an organictitanium complex selected from the group consisting of titaniumα-hydroxycarboxylic acid salt, titanium polyol complexes, titaniumalkanol amine complexes, and combinations thereof.
 21. The method ofclaim 1, 7, 12, or 17 further comprising introducing a cross-linkingcomposition comprising (a) an aqueous liquid, (b) a pH buffer, (c) across-linkable organic polymer, (d) a cross-linking agent whichcomprises an organic titanate, an organic zirconate, or combinationsthereof, (e) a delay agent which is a hydroxyalkylaminocarboxylic acidand (f) proppant, into the fracture.